The present invention relates to methods and compositions useful in treating crude sources, and more specifically, to methods and compositions useful in reducing the concentration of phosphorus in crude streams recovered from subterranean formations. Additionally, the invention relates to reducing volatile phosphorus in hydrocarbon flowback fluids.
“Crude” as that term is used herein refers to any hydrocarbon that is a liquid at standard conditions. An example would be unrefined petroleum or liquid petroleum. The term “hydrocarbon flowback fluid” is generally defined as the hydrocarbon treatment fluid used in a subterranean application, e.g., a fracturing operation wherein the fluid is initially injected into the oil well during fracturing, and later flowed back out of the well as a natural consequence of returning the oil well to production. A hydrocarbon flowback fluid may comprise produced fluids from the subterranean formation including, but not limited to, crude, water, and dissolved gases, which may include those hydrocarbons which exist as gases at standard conditions, such as a gas chosen from the group consisting of: methane, ethane, propane, butane, and iso-butane. A hydrocarbon flowback fluid may be considered a type of crude stream if it will be further refined.
Recently, many crude oil refiners in North America have experienced greater than anticipated equipment surface fouling of their refining units. For example, since 1995, refinery distillation tower plugging has been observed in some refineries processing Canadian sweet light crude. There also has been increased fouling of heat exchangers. Additionally, there is the potential for phosphorus to damage engines and catalytic converters if it is not removed prior to being used in a fuel stream.
Because these increased fouling problems dictate more frequent shutdowns for cleaning, the net business result can be a significant increase in the overall costs of operation. These financial consequences can have far-reaching effects, especially given the price of gasoline in today's market. Additionally, due to similar problems that have been encountered with Canadian crude, there is potential for devaluation of the crude.
It is believed that one source of this fouling is the presence of phosphorus in the crude streams and hydrocarbon flowback fluids being refined. The phosphorus components are typically present in small, but measurable, amounts (e.g., usually about 5,000 ppm or less) as a result of being injected into oil wells where the crude or the hydrocarbon flowback fluid is recovered. Hydrocarbon flowback fluids may contain about 1000 ppm or less. Crude from pipelines may contain about 30 ppm or less, with concentrations of about 2 to about 8 ppm being somewhat common.
Phosphorus may be introduced into a subterranean formation from which hydrocarbons may be produced through the use of gelled fluids that may contain phosphorus-based gelling agent systems. Fracturing the subterranean formation surrounding a well bore is thought to produce channels in the rock so that productivity can be maximized. Use of an organic fluid during fracturing usually serves to reduce problems associated with water-sensitive formations. Gelling agents serve to viscosify the fluid, allowing proppant to be suspended in the fluid so that it may be transported downhole to be placed in fractures resulting from the fracturing treatment. Gelled oil fracturing fluids that comprise phosphate ester gelling agents are examples of such fluids. Another example of a reversible gelling agent is a metal soap of a partially esterified phosphate. The viscosity of the fluid may ultimately be reduced for recovery.
The large increase in fouling rates observed over time at refineries is attributed by the industry, at least in part, to the increase in the number of wells that are fractured with these types of hydrocarbon-based phosphates. As mentioned, this has become a major concern for refiners due to consequential fouling problems. The fouling problems observed can be particularly problematic because the preferred cleaning involves a combination of mechanical and chemical methods. The use of such methods can be costly because refining distillation units usually have to be shut down to carry out the cleaning, thereby leading to lost refining production.
One method of combating the phosphorus problem is to distill the crude. However, this is thought to be not very effective. It is also expensive and logistically difficult in some cases because getting the crude and the hydrocarbon flowback fluids to a treatment facility can be cumbersome. Also, hydrocarbon flowback fluids flowback over time; therefore, the time to treat these fluids can be difficult. Other solutions involve not using phosphorus based gelling agents in subterranean treatment fluids; however, other gelling agents have not proven to work as well for oil-based fluids. Another solution would be to remove or reduce the phosphorus from the crude or hydrocarbon flowback fluids, but this has proven difficult as well.
To combat potential fouling problems, it is desirable to have reliable crude streams. Moreover, there are governmental regulations in place that are directed to limits on volatile phosphorus in crude streams that impact crude suppliers.